1. Field of the Invention
The present invention relates to a position detecting system and a position detecting method, and, more particularly relates to a position detecting system and a position detecting method for detecting a position of a capsule body-insertable apparatus (a detected object) inserted into a subject by using a magnetic field.
2. Description of the Related Art
In recent years, a capsule body-insertable apparatus (hereinafter, “capsule endoscope”) including an imaging device has been developed. The capsule endoscope is orally inserted into a subject to capture images, and wirelessly transmits acquired images (hereinafter, “in-vivo images”) to an external device disposed outside the subject. An operator can diagnose symptoms or the like of the subject by visually confirming in-vivo images received by the external device.
Generally, such a capsule endoscope cannot move in a subject by itself, and moves in the subject by peristaltic actions of the digestive system of the subject. Therefore, for example, it is inferior in observation capability as compared with an endoscope such as a fiberscope, with which an operator can select a region to observe freely to some extent.
As a technique of solving such a problem, for example, there is Japanese Laid-open Patent Publication No. 2005-245963. According to this patent document, the posture and movement of a capsule endoscope can be positively controlled from outside of a subject by applying a magnetic field (hereinafter, “guidance magnetic field”) to the capsule endoscope including a magnetic field generator such as a permanent magnet from outside of the subject.
However, to control the posture and movement of the capsule endoscope in the subject by a magnetic field applied from outside of the subject, as described in Japanese Laid-open Patent Publication No. 2005-245963, the position and orientation of the capsule endoscope in the subject need to be ascertained accurately. Hereinafter, detection of the position and orientation (posture) of the capsule endoscope is simply referred to as “position detection”.
In Japanese Laid-open Patent Publication No. 2005-245963, a resonant circuit including a coil (L) and a capacitor (C) (hereinafter, “LC resonant circuit”) is provided in the capsule endoscope, and the LC resonant circuit detects a resonant magnetic field generated due to a magnetic field provided from outside (hereinafter, “driving magnetic field”) by a sense coil provided in the external device, thereby detecting the position and orientation of the capsule endoscope. Hereinafter, a method of deriving information such as the position and orientation from the resonant magnetic field generated by applying the driving magnetic field from outside to the LC resonant circuit is referred to as “passive method”. With the passive method, there is a merit that power consumption of the capsule endoscope can be suppressed.
However, in the passive method, a sense coil in the external device also detects a driving magnetic field used for inducing the LC resonant circuit, other than the resonant magnetic field emitted from the LC resonant circuit. Therefore, to detect an accurate position of the capsule endoscope, an influence of the driving magnetic field needs to be eliminated to extract only the component of the resonant magnetic field.
Generally, a phase of the resonant magnetic field emitted by the LC resonant circuit is delayed by 90° with respect to a phase of the driving magnetic field. Therefore, conventionally, to extract only the component of the resonant magnetic field, for example, the influence of the driving magnetic field generated in a state without the LC resonant circuit is detected by the sense coil in advance, and at the time of actually detecting the position of the capsule endoscope (the LC resonant circuit), a magnetic-field component of the phase delayed by 90° relative to the driving magnetic field is extracted.